There are many applications where a response to information displayed on a CRT screen is desired. One such example is a service application where a cable television (TV) viewer can reserve a scheduled movie to be played on a home TV set. To this end, a menu of movie selections and phone numbers is provided on a TV screen. The viewer can then call the number corresponding to the desired selection to make the movie reservation. For details on this service application, one can refer to U.S. Pat. No. 4,763,191, issued Aug. 9, 1988 to Gorden et al., which is hereby incorporated by reference. Another application is in a video game arrangement where a video game is played on a personal computer or a television set. The video game is often programmed to display a menu of choices, which may be on, for example, skill level, sound effect, etc. The user is instructed to operate specific keys on a keyboard or a similar device to make a selection.
Nevertheless, the selection process in the above-described applications is time-consuming and oftentimes contributes to selection errors since a user must first look at the screen, determine a choice, and then examine a separate device to find the proper key or keys to effectuate this choice. Attempts have been made to facilitate the selection process. One such attempt relies on the use of a touch-sensitive screen. With this screen, a particular choice can be selected from a menu by touching with one's finger the area of the screen where that choice is displayed. Nonetheless, such screens are not desirable for certain system applications as they do not meet system cost objectives, or provide the necessary display resolution. In addition they require the use of specially designed monitors or modifications to standard monitors or TV sets.
There are, however, touch-screen techniques which can be applied to any monitor having a CRT, such as a TV set. Use of a probe with an inductive coil such as that described in a copending application entitled "Touch-Screen Arrangement," assigned to the present assignee, and hereby incorporated by reference, is one such example. This probe is hereinafter referred to as the inductive probe. Use of a light pen is another example.
With either the inductive probe or a light pen, a reference signal is required in order to determine the touch-position of the probe or the light pen on the CRT screen. The provisioning of this reference signal calls for the extraction of certain signal utilized within the monitor, thus necessitating an undesirable modification to the monitor's circuit. It would be desirable, therefore, if this reference signal provisioning could be accomplished without such limitations.